“Having our cake and eating it is not an option in the real world,” Donnelly will warn in a speech this week.

His comments come ahead of a major speech by the International Trade Secretary Liam Fox.

LONDON – Leaving the EU in the hope of building better trading relationships elsewhere is like giving up a lavish dinner now in the hope of a “packet of crisps later,” the UK government’s former chief trade official has warned.

Sir Martin Donnelly, who was the permanent secretary at Liam Fox’s Department of International Trade until last year, told Radio 4’s Today programme that attempting to replace the UK’s current trading relationship with the EU with new free trade deals elsewhere was like “giving up a three-course meal now for the promise of a packet of crisps in the future.”

Donnelly pointed out that three-fifths of the UK’s current trade is done with the EU and other nations the bloc has preferential trade deals with, and added that hoping to get a better deal with countries further afield was like wishing for a “fairy godmother.”

In a speech tomorrow, Donnelly will also rubbish Theresa May’s claim that Britain can retain the benefits of the single market and customs union after it has left.

“Having our cake and eating it is not an option in the real world; ‘frictionless trade’ is a phrase without legal content,” Donnelly will say.

Watch Donnelly ridicule Brexit and Liam Fox

"Brexit is like giving up a three-course meal… for the promise of a packet of crisps in the future."

Donnelly will also claim that it is inevitable that Britain will eventually return to the EU single market.

“Given the negative consequences of leaving, and the lack of any significant offsetting advantages, I believe it is likely the U.K. will seek to return to full membership of the EU single market in due course,” he will say.

“But significant damage to employment, the structure of the economy and the competitiveness of U.K. firms can be expected in the meantime.”

Outfoxed

GettyLiam Fox

Donnelly’s intervention comes ahead of a major speech this lunchtime by the International Trade Secretary Liam Fox, in which he will call for Britain to reach out to new markets rather than rely on the “structures of the past”.

“There is a tendency among some nations to cling to the ‘known’ trading mechanisms more suited to the structures of the past than the digital age of the future,” Fox will say.

“Flexibility and agility, then, are the key to any future trade policy. The ability to react quickly to new developments, to explore new opportunities and to nurture fledgling industries that will be the key to growth and prosperity in the coming years.”

Fox will attack the Labour leader Jeremy Corbyn, following his announcement yesterday that the party is backing Britain staying in a customs union with the EU after Brexit.

Corbyn’s speech was warmly welcomed by business groups yesterday.

“The Labour leader’s commitment to a customs union will put jobs and living standards first by remaining in a close economic relationship with the EU,” CBI Director-General, Carolyn Fairbairn, said.

However, Fox will dismiss this today, saying that remaining in a customs union would prevent Britain from signing new trade deals.

“The inevitable price of trying to negotiate with one arm tied behind our back is that we would become less attractive to potential trade partners and forfeit many of the opportunities that would otherwise be available to us,” he will say.